Dear Naveen, first of all about the last query. Yes it's mandatory for you to give CAT exam to be eligible for taking admission in any IIM.

Next comes about your graduation Marks. This depends on the IIM in which you are applying. Generally, 30% of total Weightage is given to your 10th/12th and graduation Marks. Many colleges give12th marks more preference over graduation Marks.

If you are planning for the top IIM then you percentile should lie above 98(in case your 10th/12th marks are not upto the mark). If you have excellent academic record then score of 96 percentile could get you a call from IIM if your luck favours.

All the best

Hope this helps